Case-hardening metal



Patented Jan. 13, '1931 UNITED STATES PATENT OFFICE ronrnn w.

= AND nnwann n. alumina, or msroN, PENNSYLVANIA, ASSIGNOBS T AMERICAN CYANAMID COMPANY, OF NEW YORK, N. Y A CORPORATION OF MAINE 80 Drawing.

This invention relates to improvements in case hardening materials and processes of case hardening metals and is acontinuation in part of our copending application Ser1al No. 200,762, filed June 22nd, 1927.

One method of case. hardening metals that has heretofore been used to a considerable extent is the immersion of the metal parts to be case hardened in a liquid bath of suitable alkaline chlorides and alkaline carbonates containing alkali metal cyanides. The use of such cyamdes is accompanied by serious d1sadvantages due to their high cost and also to the fact that they give of irritating gases from the bath and are very poisonous.

In Patents Nos. -1,27 9,457 and 1,27 9,458, both granted on September 17th, 1918, to Porter W. Shimer, one of the present applicants, there is described a molten bath con- 20 'sisting of mixtures of calcium chloride,

barium chloride and sodium chloride, potaschloride and sodium chloride and other similar compounds and mixtures. Such a molten bath is caused to acquire case hardening properties by immersing calcium cyanamid therein. The process of the Shimer patents is particularly advantageous as the eyanamid is not poisonous, is readily obtainable at a moderate cost and does not emit ir- 9 ritating and dangerous gases when immersed in a bath of molten salts. The gases evolved in the bath burn at the surface and are nonpoisonous and non-irritating.

When a case hardening bath of the character described in the Shimer patents is used for a long time, especially under crude operating conditions, it gradually loses its power of acquiring full case hardening properties by the immersion of additional calcium cyanamid therein. This loss of power to acuire full case hardening properties is sometimes augmented by a long continued use of the bath on large production scale, particularly when the articles being case hardened are coated with scale or when cast iron pots are used to contain the bath. This deterioration of the bath is accompanied by the formation of a film of inert and relatively impenetrable material over the surface of 59 the calcium cyanamid which prevents reac- CASE-HARDENING METAL Application filed February 28, 1929. Serial No. 342,299.

tion of the calcium cyanamid with the bath and thus lessens its effectiveness to impart case hardening properties to the bath. Even the addition of fresh cyanamid to the bath reagent such as sodium cyanide. When the reducing agent is added to the bath, a noticeable activity is obtained therein followed by the separation of dross which may be removed from the bath by means of a perforated ladle. The removal of these impurities fully restores the case hardening prop-' erties of the bath and such purification or renewal of the bath may be resorted to when necessary.

In the present invention-we provide a case hardening process in which the case hardening strength of the bathmay be maintained more uniform and in which the separation of dross is brought about by the addition of the case hardening'agent. Instead of renewing the case hardening strength of the bath at in.-

frequent intervals by the addition of a rela-.

tively large quantity of calcium cyanamid, we add a case hardening compound to the bath at regular and frequent intervals so that the case hardening strengthof the bath is maintained more or less constant and the impurities that would otherwise collect in the bath may be removed therefrom at the time of each addition of the case hardening compound. Instead of obtaining a slow accumu-.

lation of objectionable residues the bath is kept clear and caused to retain its casehardening properties by the regular, direct and more or less frequent addition of case hardening material. I

Various materials may be employed for imparting case hardening properties to a bath J?" small amounts of the case hardening material by our new process. We have found, however, that the best results are obtained when substances which we term cyanamid cyanides are employed. Such a cyanide may be formed by fusing calcium cyanamid with sodium chloride. In the Erlwein and Frank Patent No. 708,333, issued September, 2nd, 1902, there is described the manufacture of calcium cyanide by fusing calcium cyanamid and carbon in the presence of a sodium chloride flux. Crude calcium cyanamid,.h0wever, contains suflicient carbon as an impurity to unite with the CN radical of-the cyanamid and produce the CN radical of the cyanide. As set forth in the Erlwein andFrank patent, page 1, line 95, to page 2, line 6, and in an article by Dr. W. S. Landis entitled A New Cyanide appearing on page 265, Volume XXII, No. 6, of Chemical and Metallurgical Engineering (Feb. 11, 1920) a crude calcium cyanide can be obtained from crude calcium till cyanamid by fusingthe calcium cyanamid with sodium chloridewithout the addition of carbon. This crude calcium cyanide, which we term cyanamid cyanide, contains sodi um chloride, carbon free lime and calcium cyanamid as impurities. The cyanamid cyanide or crude. calcium cyanide, may be addedto a bath of fused salts regularly and periodically in minute proportions, such as 4 percent of the weight of the bath or V ounce to a 15 pound bath. A oath o'fothe type disclosed in the prior patents of Porter W. Shimer maybe employed, but we preferably employ a bath obtained by fusing. dry sodium chloride and dry. calcium chloride in proper.

proportions, While we have obtained good case hardening with the addition of as little as A of 1- percent of cyanamid cyanide, or crude calcium cyanide, to the weight of the .melt, we do not wish to limit ourselves to this amount and contemplate, when desirable, using additions up 'to 1 percent of the weight of the melt. The exact amount and frequency of these additions-naturally varies according to the circumstances, such as the size of the bath, the shape of the pot, the temperature, the amount of work .or articles to be case hardened, the quantity of the bath materials removed'by adherence to the articles, and the 1ike.= We have-found'that in a number of cases the regular hourly additions of 4 percent by weight of cyanamid cyanide, or

crude calcium cyanide, to the bath serve to keep the strength of the bath constant through the day with consequence uniformly of depth and hardness of case.

In accordance with the present invention we have found that the regular and more or less frequent addition to a bath of fused salts of comparatively small amounts of crude cal- 3 cium cyanide has produced excellent case hardening results. This regular addition of amid cyanide and Obtaining a crude calcium cyanide other cyanamid cyanides may be formed by fusing other alkaline earth cyanamids, such as barium cyanamid 'with suitable salts.

As a further modification of the process above disclosed, we may add the cyanamid cyanide (crude calcium cyanide) in small but regular amounts to a case hardening bath which already contains calcium cyanamid, or in other vwords, to the bath of the earlier Shimer patent. The small and frequent additions of crude calcium cyanide to such bath has the effect of considerably prolonging the life of the cyanamid previously immersed therein. The cyanamid at the bottom of the melt acts as a back log and gives off case hardening activity throughout the run while the addition of the small amount of cyanamid cyanide equalizes the activity of the bath and enables the user to obtain a much longer life from the cyanamid. In this way the effectiveness of the cyanamid is doubled and often trebled. v

It will be seen that our new process of case hardening metals has many advantages over former methods. Harder, deeper and more uniform cases may be obtained with greater economy and ease of operation. The case hardening bath retains its case hardening properties more uniformly when case hardening properties are imparted to the bath by regular and more or less frequent additions of' crude calcium cyanide. As stated above, the bath may consist of a mixture of dry fused salts to which the case hardening properties are imparted by these small but regular additions of crude calcium cyanide, or the bath may originally contain a case hardening material, such as calcium cyanamid, the activity of which is extended metal cyanide.

3. A molten bath for case hardening metals consisting essentially of a minor proportion of an alkali earth metal cyanide, an alkali earth metal halide and an alkali metal halide.

4. A molten bath for case hardening metals consisting essentially of a mixture of an alkali earth metal halide, an alkali metal halide and a minor proportion of calcium cyanide containing a minor proportion of calcium cyanamid.

5. A molten bath for case hardening metals consisting essentially of a mixture of calcium chloride and sodium chloride and a minor proportion of calcium cyanide containing a minor proportion of calcium cyanamid.

6. A molten bath for case hardening metals consisting essentially of a mixture of calcium chloride and sodium chloride and a minor proportion of calcium cyanide.

In testimony whereof we aflix our signatures. PORTER W. SHIMER. EDWARD B. SI-IIMER. 

